The in vitro T helper cell (Th) response of human peripheral blood leukocytes (PBL) to HLA alloantigens is mediated by three distinct Th-antigen presenting cell (APC) pathways: 1) CD4+ Th and autologous APC (CD4-sAPC); 2) CD4+ Th and allogeneic APC (CD4-aAPC); and 3) CD8-+ Th and allogeneic APC (CD8-aAPC). There is a hierarchy of sensitivity of these pathways to the immunosuppressive effects of cyclosporin A (CsA), such that the CD4-sAPC is the most sensitive and CD4-aAPC is the least sensitive, both in vitro and in vivo. We tested the in vitro Th function of more than 250 human renal allograft recipients on multi-drug immunosuppressive therapy. Our results indicate that only the presence of an intact CD4-sAPC pathway correlated with chronic or acute graft rejection. Our findings suggest that this approach can be used to monitor the graft status of organ transplant recipients. This pattern was modified in patients who received simultaneous and pancreas grafts in that the incidence of rejection of both organs was increased to 100%, and all three Th pathways appeared to contribute to rejection.